MRL Prostate GTV Focal Boost: Is Prep Relevant?

Purpose Delivery of a focal boost to the intraprostatic gross tumour volume (GTV) for prostate cancer patients has shown to improve biochemical disease-free survival and has been done via brachytherapy at our institution. However, brachytherapy has contraindications and patients may not want to undergo an invasive procedure. Onboard MR imaging enables visualization of gross disease and can guide treatment on a linear accelerator (linac). The purpose of this study was to describe the initial experience of delivering a focal boost to intraprostatic GTV(s) using MR-guidance on a 1.5T MR-Linac and determining the relevance of bladder preparation for this adaptive radiotherapy technique, because bladder filling may contribute to intrafraction motion. Methodology Between March 2023 and January 2024, 20 patients with prostate cancer were enrolled onto a prospective study involving the delivery of a 15 Gy in 1 focal boost to their GTV(s), followed by 30 Gy in 5 to the whole gland. GTV(s) were required to be less than 1/3 of the total prostate volume. Patients were treated using the 1.5T MR-Linac (Unity, Elekta). All patients were asked to empty their bladder prior to the treatment session. The first 10 patients were treated without additional hydration (no-prep), while the latter 10 were given 300mL of water to drink 10 minutes prior to entering the treatment room (prep). The margin expansion used to generate the planning target volume was 3mm in the left-right direction, and 4mm in all other directions from the contoured GTV volume. Daily online adaptive procedure used the full adapt-to-shape (ATS) workflow, including online recontouring and full inverse-replanning on the initial 3D T2-weighted localization image. Additional ATS or adapt-to-position (ATP) virtual-couch-shift procedures, and additional verification imaging were done when large intra-fraction motion was exhibited by the patient. If further large intra-fraction motion was observed after additional ATS or ATP procedure, the session would be aborted, and the patient rescheduled for treatment on a subsequent day. Treatment time, bladder volumes and frequency of additional adaption and aborting treatment were collected, and the student t-test was used for mean comparison. Results Delivery of the focal boost was completed for all 20 patients. Average treatment time was 55 minutes (sd = 9.8minutes). Bladder dose constraint D0.05cc<1800cGy was met for all patients. Mean bladder volumes on the initial localization scans were not statistically different between the two groups (no-prep: mean=87.4cc, SD=50.7cc vs prep: mean=134.8cc, SD=64.7cc, p = 0.0855). Of the 10 no-prep patients, additional adaptation was required for 4 patients and treatment was aborted for 3 due to large intrafraction motion that could not be rectified with a second adaptation. Although additional adaptation was performed for more patients treated with prep (n=6), all patients received treatment on the scheduled day. Conclusions Whilst bladder volume between patients with and without prep were not statistically significantly different, based on the current data, bladder prep is still relevant to increase the likelihood of successful treatment delivery. Further investigation is required to understand what other factors may have contributed to the large intrafraction motion observed that led to treatments being aborted.